A method is described in patent references 1 to 3 and non-patent references 1 to 3, in which a GaN layer is epitaxially grown on an Al2O3 substrate, and the back surface of the Al2O3 substrate is irradiated with a pulse laser to decompose GaN near the interface between the Al2O3 substrate and the GaN layer, thereby separating the GaN layer from the Al2O3 substrate (this method will be referred to as a laser lift-off method hereinafter).    [Patent Reference 1] U.S. Pat. No. 6,559,075    [Patent Reference 2] U.S. Pat. No. 6,071,795    [Patent Reference 3] Japanese Patent No. 3518455    [Non-Patent Reference 1] O. Ambacher et al., Materials Research Society Symposium, Vol. 617 (2000), pp. J1.7.1-J1.7.12    [Non-Patent Reference 2] W. S. Wong et al., Applied Physics Letters, Vol. 75, No. 10, 6 Sep. 1999, pp. 1360-1362    [Non-Patent Reference 3] D. Morita et al., Japanese Journal of Applied Physics, Vol. 41 (2002), pp. L1434-L1436
In the laser lift-off method, when the GaN layer is separated by pulse laser irradiation, the Al2O3 substrate can crack due to the pressure of N2 gas generated by decomposition of GaN. This can cause damage such as microcracks to the GaN layer. The microcracks cause degradation in characteristic or a decrease in yield of devices formed later in the damaged GaN layer.
In the laser lift-off method, separation takes a long time because the whole surface of the substrate must be scanned by the pulse laser.
In the laser lift-off method, a three-dimensional pattern as a trace of pulse laser scanning is generated on the separated surface of the GaN layer. To remove this three-dimensional pattern, an additional step such as polishing is necessary. This makes the operation cumbersome. In addition, the increase in number of manufacturing steps can lead to a decrease in yield.
In the laser lift-off method, the substrate used to grow a semiconductor film is limited to a transparent substrate such as an Al2O3 substrate that passes a laser beam. It is hence difficult to apply a nontransparent substrate such as an SiC substrate, GaAs substrate, or Ge substrate.
As described above, to introduce the substrate separation method by the laser lift-off method to mass production of semiconductor devices, still more technological development to, e.g., increase the yield has been demanded.